Add option to rename columns during selection

[juliohm]

A nice feature of the DataFrames.jl select function is that it allows in-place renaming:

select(df, :x => :y, :a => :b)

Currently, we have to write a Select(:x, :a) and then use Rename(:x => :y, :a => :b) on the result. Maybe we could introduce a SelectRename transform or generalize the Select to accept renaming.